The present invention broadly relates to a new and improved construction of an apparatus for detecting and evaluating infrared radiation.
In its more specific aspects the present invention relates to a new and improved construction of an infrared intrusion detector, comprising at least one aspherical optical arrangement for bundling the infrared radiation from at least one strip-shaped region or zone of incident radiation directed onto a sensor, and further including an evaluating circuit connected to the sensor for generating an output signal as a function of a predetermined amount of radiation change at the sensor.
In other words, the infrared intrusion detector of the present invention is of the type comprising an infrared sensor for sensing infrared radiation, at least one strip-shaped reception zone for receiving the infrared radiation, at least one aspherical optical arrangement for focusing the infrared radiation on the infrared sensor, and an evaluation circuit connected to the infrared sensor for emitting a signal in response to a predetermined alteration in the irradiation of the infrared sensor by the infrared radiation.
Such intrusion detectors are known, for instance, from U.S. Pat. No. 4,058,726, granted Nov. 15, 1977, or German Patent No. 2,645,040, granted Oct. 9, 1981. FIG. 4 of each of these patents depicts an intrusion detection apparatus. Strip-shaped sectorial zones of the field of reception are formed by cylindrical lenses, one lens per sector, arranged in front of a detector. This method is quite suitable for keeping a room under surveillance by providing a number of vertically oriented, parallel, flat field sectors or zones, which will reliably trigger an alarm upon their being crossed by an intruder.
Such known arrangements however, are limited in effectiveness when employed in applications requiring a single sector or zone, or sectors or zones not in close proximity with each other, yet having accurately defined border lines, such as required for protective curtains in front of openings, such as doors or windows, or as "flat gates" in front of protected objects. First of all, the vertical incidence angle is limited by the housing, and virtually never approaches the nearly 90.degree. required for a fully effective protective curtain. Furthermore, only the center portion of the cylindrical lens can provide proper focusing, since only in this particular location does the sensor lie within the focal area. For eccentrically incident radiation, the sensor lies outside the focal area, resulting in a defocused or blurred image. As a result, offset or eccentric incident radiation produces diffused border lines and, under extreme conditions, adjacent areas are likely to stray into the neighboring reception zone, so that efficient protection becomes impossible.
A further disadvantage lies in the increasing length of the optical path with increasing off-center or eccentric orientation of the incident angle inherent in cylindrical lenses. Since the far infrared absorption of the material of the lens is no longer negligible for instance in the 10.mu. range (peak body radiation of humans), radiation attenuation becomes more pronounced with increasing off-center or eccentric orientation of the incident radiation. Such an intrusion detector therefore loses sensitivity with increasing scope of the incident angle, and thus further limits the usefulness of such an arrangement for a protective curtain type application.
The intrusion detector described in U.S. Pat. No. 4,375,034, granted Feb. 22, 1983 for one or more receiving regions or zones, shows an arrangement with improved borderline definition and increased aperture or reception angle. It includes a spherical reflector, comprising a special arrangement of one or more cylindrical reflectors. This requires a complicated optical arrangement, with precise adjustment of reflectors with respect to each other, and this precise adjustment must be maintained throughout the lifetime of the apparatus (i.e., many years). Furthermore, reflector surfaces tend to age, and become soiled in time, causing a progressive loss of reflectivity together with increased scattering, so that the sensitivity, efficiency and operational reliability of this type detector steadily decrease.